The most useful data that's looked at includes visits/visitors, bounce rate, time-on-site and sources. As mentioned above, it's vital for marketing, reports and optimization of software or pages. Visits and visitors are counted and recorded to calculate the ratio for new visits against returning visitors. Bounce rate determines how relevant or effective your site is to captured audience. Time-on-site is pretty self-explanatory, while sources covers where visitors came from (be it search engines, direct or referring sites).
Unfortunately, since Google Analytics makes use of Flash for graph rendering, the service is less-than available on smartphone platforms that do not support Flash. How would a Windows Phone user keep up with site statistics while on the go? Allow me to introduce Phonealytics, by Luke Lowrey.